We studied the safety of immunotherapy with an Alternaria extract in patients with rhinitis and bronchial asthma. The few studies that have investigated immunotherapy with mold allergens suggest that they cause adverse reactions more frequently than do other extracts. All treatments prescribed by our allergology service with Alternaria immunotherapy between 1988 and 1996 were recorded and analyzed. In all cases a biologically standardized depot extract of Alternaria tenuis containing 5 BU/ml was used according to a conventional immunotherapy schedule. During the study period 129 patients received immunotherapy with Alternaria extract. Of the 3,892 doses given, 1.95% led to adverse reactions, which occurred in 39.5% of the patients. Most of the adverse reactions were systemic and mild, and reproduced the underlying disease. The risk of adverse reactions was significantly higher in children, patients with asthma, and during the initial phase of treatment. Patients who suffered from adverse reactions had a significantly higher level of total and specific IgE. It was concluded that tolerance of Alternaria mold extract was worse than for other allergenic extracts, although most reactions recorded were mild. The risk of adverse reactions was greater in children and patients with asthma, and during the initial phase of immunotherapy.